A sintered bearing is generally used under a state in which inner pores thereof are impregnated with a lubricating oil. In this case, along with rotation relative to a shaft inserted into an inner periphery, the lubricating oil contained in the inner pores seeps out onto a sliding portion with the shaft. Then, the seeping lubricating oil forms an oil film, and the shaft is supported by the oil film so as to be freely rotatable relative to the sintered bearing. The sintered bearing (oil-impregnated sintered bearing) described above is assembled into, for example, a power transmission mechanism for a power window, which is configured to open and close window glass of a vehicle for use.
The power transmission mechanism for a power window mainly includes, for example, as illustrated in FIG. 24, a motor 261, a shaft 262 to be rotated by the motor 261, a worm gear 263 provided on the shaft 262, and a wheel gear 264 configured to mesh with the worm gear 263. Rotational power input from the motor 261 to the shaft 262 is transmitted to the wheel gear 264 through intermediation of the worm gear 263 under a speed-reduced state, and is further transmitted to a window glass opening and closing mechanism (not shown). The shaft 262 is supported by a plurality of bearings 265 arranged in a separated manner in an axial direction of the shaft 262 so as to be freely rotatable with respect to a housing 266. As each of the above-mentioned bearings 265 configured to support the shaft 262, the sintered bearing (oil-impregnated sintered bearing) is suitably used.
In the power transmission mechanism illustrated in FIG. 24, a load F in a direction orthogonal to an axis of the shaft 262 is applied to a part of the shaft 262 (part thereof in a longitudinal direction of the shaft 262) as a result of meshing between the worm gear 263 and the wheel gear 264. Therefore, the shaft 262 is warped. In this case, the part of the shaft 262 is rotated relative to the bearings 265 under an inclined state with respect to an axis line of the sintered bearings 265. Therefore, an outer peripheral surface of the shaft 262 locally slides on inner peripheral surfaces (bearing surfaces) of the bearings 265. As a result, there is a fear of occurrence of disadvantages such as wear of the bearing surfaces and generation of abnormal noise.
In view of the disadvantages, in the power transmission mechanism described above, there has been examined use of sintered bearings disclosed in, for example, Patent Literature 1 and Patent Literature 2, specifically, a sintered bearing including, on an inner peripheral surface, a cylindrical portion and an increased-diameter portion (one-side increased-diameter portion) which is provided so as to be adjacent to one axial side of the cylindrical portion and has a diameter gradually increased toward the one axial side. Specifically, when the above-mentioned sintered bearing is assembled into the power transmission mechanism so that the increased-diameter portion is arranged on a side closer to the worm gear 263, the outer peripheral surface of the shaft 262 can be supported by the increased-diameter portion of the sintered bearing even in a case where the part of the shaft 262 is warped. Therefore, stress concentration on the inner peripheral surface of the sintered bearing can be relaxed. Thus, occurrence of the above-mentioned various types of disadvantages can be prevented as much as possible.
The above-mentioned sintered bearing including the cylindrical portion and the increased-diameter portion on the inner peripheral surface can be obtained by performing sizing on an inner peripheral surface of a cylindrical sintered compact, for example, as disclosed in Patent Literature 3. Specifically, first, a cylindrical portion molding surface formed on a first core rod, which corresponds to a shape of the cylindrical portion, is pressed against the inner peripheral surface of the sintered compact (region of the inner peripheral surface, on which the cylindrical portion is to be molded), thereby molding the cylindrical portion. Thereafter, an increased-diameter portion molding surface formed on a second core rod, which corresponds to a shape of the increased-diameter portion, is pressed against, that is, is gradually pushed into a region of the inner peripheral surface of the sintered compact, on which the increased-diameter portion is to be molded, thereby molding the increased-diameter portion.
Further, in the sintered bearing disclosed in Patent Literature 2, a density of the increased-diameter portion is increased to reduce a surface opening ratio. In this manner, penetration of the lubricating oil from the increased-diameter portion into the inner pores is suppressed to enhance a function of supporting the shaft. A density of the inner peripheral surface (including the cylindrical portion and the increased-diameter portion) of the sintered bearing is adjusted by increasing or decreasing an amount of compression in a sizing step.